[cifs-protocol] [EXTERNAL] [MS-OAPXBC] Incorrect session key instructions

[David Mulder]

On 1/25/24 9:45 AM, David Mulder wrote:
>
>
> On 1/25/24 9:41 AM, Sreekanth Nadendla wrote:
>> Hello David, the data obtained after base64url decode is the key in 
>> encrypted form. You would use the machine transport key to decrypt 
>> this to obtain the pop key you need. Can you check if this works?
> That's exactly the problem. It can't be decrypted using the transport 
> key. The number of bytes is too long. The CEK returned by MS appears 
> to be corrupted (or a buffer overrun on your side, we're not sure).

For example:

 > python3

 >>> from cryptography.hazmat.primitives import serialization
 >>> from cryptography.hazmat.primitives.asymmetric import padding
 >>> from cryptography.hazmat.primitives import hashes
 >>> transport_key = serialization.load_der_private_key(b'<redacted>', None)

 >>> session_key_jwe = 
"eyJlbmMiOiJBMjU2R0NNIiwiYWxnIjoiUlNBLU9BRVAifQ.Lwx1oUwtrOVhZoHkPlNCVfvmInTIVfkpY4daNtS7fiL-dL-G2pgnSbCG23vwmk8VF9dbQPKkN4ERiWsXA8hjaZPE4XcWsylUrbT65hyO3U_r3nXLGxAYX06rRP21L8ak1qoFAl9wodJI30yHmBqYdsrO3BNa0QRXNmvliRF1fNnvzuRj5VQiqFi78-8as7rwKtUQ117R11q3EvaoYgwQUJS1JdDAiRDRHuVpVmfH8Gf279EpRuhKlyEN1gtjXCcK1U9cj3Oco47JeS3AuCZOrU0Q0rRSt0hWBFC21mLxqQ64hXTG3NOb5O-DFoN7sIf7vDBdQloZ2Sxq5gDVdegfmcsKTnjD3nooJIOuT8mmCyTeqdHlio-sYNBm0QzSsLPP3Dngl1bK.yLJM5ZkeigtBz5Cl.TA.lBRRBpOedY0K62Ti7jDqNA"

 >>> encKey = base64.urlsafe_b64decode(session_key_jwe_parts[1]+'==')
 >>>
 >>> transport_key.decrypt(encKey, 
padding.OAEP(mgf=padding.MGF1(algorithm=hashes.SHA1()), 
algorithm=hashes.SHA1(), label=None))
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File 
"/usr/lib64/python3.11/site-packages/cryptography/hazmat/backends/openssl/rsa.py", 
line 444, in decrypt
     raise ValueError("Ciphertext length must be equal to key size.")
ValueError: Ciphertext length must be equal to key size.

-- 
David Mulder
Labs Software Engineer, Samba
SUSE
1221 S Valley Grove Way, Suite 500
Pleasant Grove, UT 84062
(P)+1 385.208.2989
dmulder at suse.com
http://www.suse.com
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